The invention pertains to a coating composition comprising a first compound comprising at least one bicyclo-orthoester group and a second compound comprising at least two isocyanate groups.
The above-mentioned coating composition is known from WO 97/31073. It has been found that with the application of such a coating composition as a clear coat on certain substrates foam formation is observed at relatively low film thickness.
The invention now provides a coating composition of the aforementioned type which is free of said drawback. For that reason the coating composition mentioned in the opening paragraph is wherein the it comprises a third compound comprising at least one thiol group.
The use of thiol-functional compounds in the coating composition of the present invention provides coatings with little or no foam formation and a film thickness up to at least about 60 xcexcm without film defects. Such a layer thickness of at least about 60 xcexcm will result in characteristics which, as known to a person skilled in the art, are preferred for a clear coat, such as good protection of the underlying coating layer(s), gloss, hardness. Furthermore, coating compositions according to the present invention show a low VOC, a favourable pot life:drying time ratio, reduced sagging of the coating composition on the substrate, and a high build behaviour.
A coating composition comprising a compound comprising at least one bicyclo-orthoester group (bicyclo-orthoester hereinafter is abbreviated to BOE) is a composition having latent hydroxyl groups. In the presence of water or moisture from the air the BOE groups will be hydrolysed, forming hydroxyl groups. This reaction is also known as deblocking. During deblocking few if any volatile components are released. When the BOE group is deblocked in this manner, it is not possible to obtain a homopolymer of BOE groups by cationic polymerisation. However, the deblocked hydroxyl groups can react with the isocyanate groups of the second compound to give a cross-linked polymer.
U.S. Pat. No. 4,788,083 discloses a coating composition comprising a polyol polyisocyanate mixture and a molar excess of a thiol-functional compound to a tin catalyst. By molar excess is meant that sufficient thiol-functional compound is added to the tin catalyst for the pot life in an open can of the polyol polyisocyanate mixture to be at least twice as long as for the same mixture containing only the tin catalyst.